Certain offshore regions that have large petroleum resources, such as the east coast of Canada, also suffer from the threat of drifting icebergs. The movements of these icebergs due to currents and wind are erratic and difficult to predict, and their size varies considerably. Based on records kept over recent years, it has been predicted that an iceberg with a mass of 5.times.10.sup.6 tonnes with a velocity of 1 m/s can be expected. The kinetic energy of such an iceberg presents a difficult problem in offshore engineering.
Although the petroleum industry has acquired considerable expertise in building structures to withstand the environmental loading due to the ocean waves, wind and current, there has not been, to date, an entirely satisfactory method of dealing with drifting icebergs.
The damage which may be caused by an iceberg is not only the potential destruction of an oil exploration and production platform, but also damage to all the ocean bed mounted facilities such as well heads, pipes, manifolds and tanker mooring facilities. In addition to the loss of capital equipment and disruption to exploration and production, there may also be environmental damage.
Proposals to date for development of petroleum resources in iceberg infested waters include one of the following: One approach is to monitor iceberg activity so that smaller icebergs can be towed away, or the production vessel itself can be moved. Another approach is to install a relatively massive fixed production platform that can withstand the iceberg impact forces. However with either of these approaches, the icebergs remain a threat to the tanker loading facilities and subsea installations such as production manifolds and flow-lines, as well as the platform itself.
Several approaches have been proposed to provide iceberg impact protection for a single platform. One approach is disclosed in U.S. Pat. No. 4,215,952 which proposes a compliant surface attached to the below water surface of a platform. Canadian Patent No. 1,222,383 describes a system of chains attached from the top of a platform to either the bottom of the platform or to the sea bed. Some of these tension lines are described as attached to energy dissipating arrestors, which are placed on the deck of the platform. An impacting iceberg is claimed to collide with the tension lines first, thereby dissipating some or all of its kinetic energy. Canadian Patent No. 1,232,768 discloses a system consisting of a large diameter, axially stiff, buoyant annular structure which surrounds an offshore structure to be protected. The "lightly" buoyant ring is attached to the base of the platform by means of mooring lines which are "downwardly inclining towards the platform". Heavy ballast weights are attached to the ring which rest on the sea bed. During the impact of an iceberg against the annular structure, the ring will drag the ballast weights across the sea bed and may cause them to be lifted. This is purported to cause energy dissipation. The restraining forces due to the collision are taken up by the base of the platform. Another system, which is similar but does not claim protection against icebergs, is disclosed in U.S. Pat. No. 4,470,724. This patent describes a "tying system for mooring lines" of an offshore transfer terminal to avoid collision between a tanker and the floating, moored offshore terminal.
It would be desirable to provide iceberg protection for a limited area rather than to construct a specially designed iceberg resistant platform. Such special purpose platforms are significantly more expensive and they cannot provide protection to all of the submarine installations which typically surround the operation site. Furthermore, there are many conventional tankers and platforms available which could be adapted to production and placed into uninterrupted operation if suitable means can be found to protect them from the hazards of iceberg collision.
It would also be desirable to provide a system capable of restricting vessels, for example oil tankers, to navigable routes.